In printing pictorial matter, whether by lithography, letterpress or gravure, the halftone process is used, wherein the actual printing image is composed of thousands of minute dots per square inch of a single colour ink of varied dot size or ink density. What the naked eye sees as shading in half-tone prints is actually a controlled variation in size of dots relative to the unprinted areas between the dots or a controlled variation in the ink density of the dots. In black and white pictorial matter the dots are printed in black ink only. Full colour reproductions, however, are necessarily printed in each of three colours, cyan, magenta and yellow (known as "three colour process"), or in these same colours with the addition of black ("four colour process"). For each colour a separate printing plate is made. In order to make the three or four printing plates, the original colour picture or photograph is "separated" photographically or by laser scanner, wit the use of filters, masks, etc., into a set of three or four half-tone negatives or positive transparencies, each representing one of the colours, and containing, dot for dot, the amount of that colour which must be printed in order for composite three or four printed colours to produce the desired total colour print.
The preparation of the colour-separation negative or positive films from colour transparencies is an art and requires considerable skill in handling the many variables to produce a desired result. Often trial and error is involved requiring correction or replacement of one or more of the negatives or positives. Unless some reliable system is available to "proof" the colour separated films, the printing press must be set up and copy printed just to secure preliminary proofs. This is time consuming and expensive.
An example of a negative pre-press proofing system is disclosed in U.S. Pat. Specification No. 3 671 236. This proofing system is commercially available from Minnesota Mining and Manufacturing Company under the trade mark Matchprint. The proofing sheets consist of a carrier sheet provided with a release surface, which may either be a smooth surface of the carrier itself, or a surface coating thereon. Overlying the surface and in intimate clinging engagement therewith, but not adhesively bonded thereto, is a colour coating formed, for example, of a pigmented organophilic water-insoluble solvent-softenable resinous polymer. Coated over and in contact with the colour-coating is a light-sensitive diazo resin layer. The colour coating and light-sensitive layer are intimately associated and adherently bonded together (and in certain constructions can actually be combined to a single layer). The light-sensitive layer is rendered insoluble on exposure. A solution which softens and/or partially dissolves the colour coating is thereafter used to remove the unexposed areas.
Overlying the light-sensitive layer is a continuous solvent-resistant resinous protective film or layer to the exposed surface of which is applied a very thin layer of adhesive, e.g. pressure-sensitive adhesive The outer pressure-sensitive surface of the adhesive can be protected from contamination by dirt or grease, and from adhering to an adjacent sheet in a pile by a protective release liner.
In use the protective liner is first stripped from the adhesive surface and the entire structure is laminated, for example, by rolling, onto a substrate, e.g. white paper. Thereafter, the carrier sheet is stripped from the structure, the bond to the paper and the adhesion between the several layers being greater than the non-adhesive clinging engagement between the carrier sheet and the colour coating. Following the removal of the carrier, the remaining structure, now bonded to the substrate, is exposed to light through the appropriate colour-separation negative corresponding to the colour of the coating. In the light struck areas, the light passes through the colour coating (which is transparent thereto) and exposes and insolubilises the light-sensitive material. A firm bond between the light-reacted material and the under-and over-lapping coatings occurs. The non-exposed areas remain light-sensitive.
Thereafter the sheet is processed with processing solvent selected with respect to the particular material of which the layer is composed (and which contains a solvent for the unexposed diazo) to develop the image, e.g. aqueous alcohol. The colour coating and the sensitiser in the non-light struck areas are removed, leaving the colour image in areas anchored to the underlying layer by the light-reacted diazo in exposure areas. The protective layer serves as a barrier which protects the substrate (and adhesive) from solutions used during the processing.
Following the above described photomechanical production of the first colour image on the substrate, for example cyan, similar sheets but containing the yellow, magenta and black colour coatings are successively applied and the images produced over the structure to yield a four-colour proof.
U.S. Pat. Specification No. 4 260 673 discloses a positive pre-press proofing system which is commercially available from Minnesota Mining and Manufacturing Company under the trade mark Matchprint. These colour proofing sheets consist of a carrier sheet having a smooth release surface; a colour coating of a diazo oxide and a pigmented-resin compound in clinging engagement with but not adhesively bonded to said release surface of said carrier sheet; a binder layer comprising a mixture or reaction product of a resin and a diazo oxide bonded to the surface of said colour coating; said binder layer being free of colour pigment; and said colour coating and said binder layer being solubilisable in a solvent developing medium upon exposure to actinic radiation but not solubilisable in the developing medium prior to exposure to actinic radiation; and a clear barrier layer firmly attached to said binder layer, said barrier layer being insoluble in said solvent development medium.
In order to obtain a multi-coloured colour proofing sheet on one substrate, the barrier layer of a first presensitised sheet of the aforementioned construction is bonded to a substrate and the carrier sheet removed. The substrate with the presensitised proofing sheet bonded thereto is then exposed to actinic radiation through a colour separation positive corresponding to the pigment of the colour coating. The exposed diazo oxide and resin mixture in the binder layer and the colour coating is rendered soluble to a solvent developing medium to create a latent image.
The latent image may then be developed with a solvent developing medium, e.g. an aqueous alkaline medium, preferably sodium hydroxide solution, whereby the exposed diazo oxide and resin mixture in the binder layer and associated colour coating is removed leaving the unexposed diazo oxide and resin. By this removal of the exposed binder layer, the pigment normally associated with causing background discolouration which would have remained bonded to the clear barrier layer is removed.
The aforementioned process is repeated in sequence and in register so that the composite proof contains coloured layers representing the magenta, cyan, yellow and black contributions of the original subject matter.
A modified positive-acting pre-press proofing system is disclosed in European Patent Specification No. 0115899A. This proofing system may be utilised in a similar manner to that disclosed in United States Patent Specification No. 4 260 673 but the photosensitive proofing sheets normally comprise three layers and do not require the presence of a barrier layer. In particular, the positive-acting colour proofing sheet comprises a strippable carrier layer optionally having a release coating on one face thereof upon which is coated a first layer of a base-soluble organic polymeric binder layer having a diazo oxide compound or diazo oxide pendant group on the polymer binder present as at least 15 percent by weight of the binder and sufficient colourant to provide a transmission optical density of at least 1.0, and a thermal adhesive second layer comprising a thermally softenable polymeric adhesive layer, the first layer having at least a substantially polar solvent system with less than 20 percent by volume of all solvents of a non-polar solvent, and the second layer having at least a substantially non-polar solvent system with less than 20 percent by volume of all solvents of a polar solvent.
The photosensitive layer comprises a base soluble organic polymeric resin binder having an o-quinone diazide or other positive-activing diazo oxide mixed with, dissolved in, or pendant to the binder. The layer also contains colourant such as a pigment or dye. The diazo oxide must be soluble or otherwise present in amounts of at least 15% by weight in the organic polymeric resin binder. Preferred polymers for this layer are phenolic resins including both resols and novolaks. Base soluble acrylic resins are also useful. The solvent system used during the coating of the photosensitive layer (and the other layers) can have a dramatic effect. Polar solvents, or at least substantially polar solvent mixtures should be used in the coating of this layer. Examples of useful polar solvents include ketones (methyl ethyl ketone, methyl isobutyl ketone, etc.), alcohols or ethers (glycol monomethyl ether, ethoxy ethyl acetate), halogenated hydrocarbons (trichloroethane, etc.), esters (ethyl or butyl acetate), and mixtures of those polar solvents, together or with small amounts (less than 20% by volume, preferably less than 10% by volume) of non-polar solvents such as intermediate length hydrocarbons (pentane, hexane, heptane, octane, etc.). These mixtures of small amounts of non-polar solvents and polar solvents constitute the substantially polar solvent mixtures referred to above.
The base-soluble organic polymeric binder (i.e. soluble or reasily dispersible in aqueous alkaline solution of pH 13) for the photosensitive layer, as indicated above, is preferably a phenolic resin. Other polymeric materials (such as Butvar resin to disperse to hold the colourant) may be dissolved or mixed with that primary resin binder constituent. Epoxy resin may also be mixed or dissolved with the binder to adjust physical properties. Furthermore, crosslinked epoxy or polyurethane resin may be added as may polyisocyanates (or other epoxy resin crosslinking agents) and monoisocyanates to react with the phenolic resin or other polymer constituents.
The thermal adhesive layer comprises an organic polymeric, thermoplastic binder which is not developed (dissolved or softened so as to be readily removed by hand-rubbing with a developer pad) in an aqueous, alkaline solution of pH 13. Any thermoplastic resin softenable at a temperature of less than 200.degree. C., preferably at less than 175.degree. C. and most preferably at a temperature less than 160.degree. C. (with a preferred range between 100.degree. and 160.degree. C.) is useful as the binder component of this layer if it is not soluble in an aqueous alkaline developer. Preferred polymers are not tacky at 20.degree. C. and include, for example, polyenes (styrene, butadiene, acrylonitrile polymers, copolymers and terpolymers), acrylics (e.g. methylmethacrylate, n-butyl acrylate, copolymers thereof, etc.), etc. These polymers are coated out in a solvent which is not completely miscible (and preferably immiscible) in the coating of the photosensitive layer and is preferably a non-polar solvent or at least substantially non-polar solvent. Solvents such as non-polar aromatic hydrocarbons (toluene, benzene and the like), and aliphatic hydrocarbons (e.g. intermediate length hydrocarbons such as pentane, hexane, heptane, octane, etc.) are particularly preferred. To the extent that any non-polar solvents are used in coating the photosensitive layer with substantially polar solvent mixtures, the non-polar solvents used in the coating of the thermal adhesive layer may be different from the non-polar solvent in the photosensitive layer. Substantially non-polar solvents, as the term is used herein allows for less than 20% by volume of a compatible polar solvent and preferably less than 10%.
U.S. Pat. Specification No. 3 649 268 discloses an alternative colour proofing system which is commercially available from E.I. du Pont de Nemours and Company under the trade name Cromalin Positive Proofing material. In this process an element having a removable support and a photohardenable layer is laminated to a suitable receptor, imagewise exposed through a positive transparency of the original through the support to actinic radiation which selectively raises the stick temperature of those areas receiving the radiation. The support is stripped from the layer and the outer surface of the layer treated (e.g. dusted) with a colourant material which adheres only to the underexposed areas of the layer to read out the image. By repeating the laminating, exposing with separate colour separation records, stripping and treating steps in sequence using different colours, a multicolour image can be obtained.
U.S. Pat. Specification Nos. 4 247 619, 4 489 153 and 4 489 154 disclose a peel-apart colour proofing system. Such a process for preparing a positive overlay comprises:
(a) exposing through a separation positive image a peel-apart photosensitive element comprising in order from top to bottom (1) a strippable cover sheet comprised of a polymeric film which is transparent to actinic radiation, (2) a photoadherent layer containing a colourant and comprising a photohardenable material with ethylenically unsaturated or benzophenone type groups, (3) a nonphotosensitive organic contiguous layer, and (4) a sheet support, wherein after exposure to actinic radiation, the peel force required to remove the cover sheet (1) with the exposed photoadherent layer (2) thereon from the contiguous layer (3) is at least four times the peel force required to remove the cover sheet (1) from an unexposed photoadherent layer (2);
(b) peeling apart the exposed photosensitive element to form two elements (i) cover sheet bearing on its surface coloured exposed image areas, and (ii) sheet support bearing the contiguous layer having on its surface complementary coloured unexposed image areas;
(c) adhering element (ii) to a transparent receptor;
(d) removing the transparent receptor and the coloured unexposed image areas adhered thereto;
(e) repeating steps (a) to (d) at least one time; and
(f) overlaying the images in register
The process for preparing a positive overlay as described is a dry process, in which liquids need not be used. The photoadherent layer, which is the sole photosensitive layer of the element, is one whose adhesive relationship between the cover sheet and the contiguous layer is altered by exposure to actinic radiation, so that after exposure the exposed areas of the photoadherent layer adhere more strongly to the cover sheet than to the contiguous layer and are removed with the cover sheet while the unexposed areas of the photoadherent layer adhere more strongly to the tacky contiguous layer than to the cover sheet.
In a modified process the photoadherent layer does not contain colourant and the contiguous layer is receptive to colour toner which is applied after peeling apart the element.
In imaging processes, such as those described above, where a sequence of imaged layers is built up, absorption or reflection of actinic radiation by previously imaged layers can produce detrimental effects in the quality and fidelity of reproduction of images, particularly dots, upon imaging exposure. The prior art describes the use of (a) an antihalation layer used below a photohardenable layer or (b) compounds (frequently dyes) used within the photohardenable layer which absorb actinic radiation to prevent halation effects. In spectral regions where a photoinitiator strongly absorbs actinic radiation, the photoinitiator itself functions as an antihalation agent. In spectral regions where a photoinitiator moderately or weakly absorbs, e.g. in the spectral region overlapping near ultraviolet and visible blue, an additional ultraviolet absorbing antihalation agent is needed. Such ultraviolet absorbers, e.g. 2,2'-dihydroxy-4-methoxy-benzophenone, however, of necessity, tend to tint the photohardenable material being protected.
U.S. Pat. Specification No. 3 854 950 discloses a photosensitive element suitable for use in the Cromalin system which comprises a support, and at least one photohardenable layer comprising an intimate mixture of: (a) a photohardenable material, (b) a photoinitiator activatable by actinic radiation which is substantially in the near-UV wavelength spectral region (i.e. about 325 to 425 nm), and (c) a sufficient amount of an ultraviolet radiation absorbing material to reduce the actinic ultraviolet radiation transmitted by the photohardenable layer by more than about 50 percent, the photohardenable layer visually transmitting or reflecting virtually white light. The UV absorbing material must not tint the composition, and if the composition is discoloured due to other components, the ultraviolet radiation absorbing material must brighten the composition so as to render it clear and colourless, whereby the composition will visually transmit or reflect virtually white light.
The Dupont Positive Cromalin materials are generally exposed through a Kokomo filter to cut off emissions above about 425 nm otherwise the exposure time for the photosensitive material becomes so short that it is difficult to control the exposure conditions. Whilst the Kokomo filter absorbs in the near-visible spectrum it does not behave as an antihalation system as it is not in optical contact with the photosensitive element.
The commercially available colour proofing systems employing the lamination techniques described above tend to be sensitive to actinic radiation towards the ultraviolet and in particular within the general wavelength band 325 to 450 nm. These materials are normally exposed to a "photopolymer" source primarily emitting from 320 to 390 nm but also in the visible region of the spectrum up to 450 nm. Other exposure sources which may be employed include "diazo" sources which emit primarily from 385 to 435 nm and also has an emission at about 560 nm and combination photopolymer-diazo sources having primary emissions within the range 325 to 380 nm and also at about 425 nm.
Dyes suppressing halation need to be matched to the light source and the absorbance of the photosensitive layer. It is desirable to provide presensitised sheets for colour proofing which achieve halation suppression in the ultra violet and near visible spectrum.
The antihalation dyes used in the colour proofing systems of the prior art impart a residual stain or tint to the colour proofing sheet and/or do not achieve antihalation suppression throughout the wavelength band 325 to 450 nm. Furthermore, the selection of suitable antihalation dyes has been severely restricted in order to avoid imparting undesirable residual colour to the colour proofing sheets.
The present invention describes constructions of presensitised sheets for use in colour proofing systems incorporating antihalation layers which may readily be completely removed from the colour proofing sheet during processing.